(1) Field of the Invention
The present invention generally relates to carbon fiber-filled composite materials and their processing methods. More particularly, this invention relates to such composites with very small diameter carbon fibers dispersed in a polymer matrix, and in which glass fibers are dispersed and oriented to promote alignment of the carbon fibers, with the effect that electrical resistivity of the composite is reduced.
(2) Description of the Related Art
Composite materials of filler materials dispersed in a polymeric matrix are known to exhibit mechanical properties such as stiffness, strength and toughness, and physical properties such as coefficient of thermal expansion and electrical and thermal conductivities, which are superior to the polymeric matrix alone. An example is carbon fiber-filled polymer-based composite materials that have found use in both the automotive and aerospace industries due to their ability to exhibit desirable mechanical, electrical and thermal properties. To increase fiber strength, carbon fibers may undergo a thermal treatment referred to as graphitizing, such that the fibers are in the form of graphite.
Carbon fibers are often produced by the pyrolysis of long polyacrylonitrile (PAN), pitch or rayon-fibers. In this method, a suitable PAN, pitch or rayon is produced as a continuous fiber that is oxidized, carbonized and perhaps graphitized to form carbon or graphite fibers. The fibers are typically very long, and therefore must be chopped to a suitable length that may vary from a few millimeters to a few centimeters or longer. Carbon fibers produced by this method are generally at least about one micrometer in diameter, and more often on the order of several micrometers or more in diameter.
U.S. Pat. No. 5,024,818 to Tibbetts et al., assigned to the assignee of this invention, teaches a method and apparatus by which carbon fibers can be catalytically grown by a vapor deposition process from hydrocarbons. The vapor-grown carbon fibers produced by this method are generally nanometer-size (i.e., less than about one micrometer in diameter), typically on the order of about 200 nm, and are significantly smaller than carbon fibers produced by conventional methods. In addition, the fibers are relatively short, with lengths typically on the order of about 40 to about 200 micrometers, and perhaps as small as five micrometers or less. Therefore, the fibers are generally too small to allow the properties of the individual fibers to be measured directly. Though their high surface area and large stiffness make them too fragile for many types of production mixers, such as twin screw extruders, vapor-grown carbon fibers of the type produced by Tibbetts et al. have been used as additives in carefully fabricated thermoplastic composites.
Electrical conductivity is a necessary property for applications such as charge dissipation, electrostatic painting, radio frequency interference and fuel cell plates. Carbon fiber-filled polymer-based composite materials can be sufficiently electrically conductive for such applications, particularly those filled with vapor-grown carbon fibers because smaller fiber diameters are able to achieve suitable electrical conductivities with only a small volume fraction of fibers added. Minimizing the fiber content of a composite reduces material and processing costs while also avoiding degradation of composite properties such as impact resistance. However, a difficulty is encountered when vapor-grown carbon fibers are incorporated using high shear bulk fabrication techniques suitable for large volume production processes. An example is a twin screw extruder whose high shearing forces tend to break carbon fibers, destroying the interconnections between fibers that are necessary for thermal and electrical conductivity through the composite. Such an undesirable effect is particularly seen with small-diameter vapor-grown carbon fibers, whose high surface area and stiffness render the fibers too fragile for many types of production mixers. The result is a composite whose electrical resistivity is significantly higher than what can be achieved with a relatively gentle low-volume mixing technique, and may exceed the allowable level for the particular application, such as about 106 Ohm×cm for electrostatic painting applications. While higher carbon fiber contents of 15 volume percent or more can reduce resistivity, such composites are more difficult to process and can exhibit unacceptable mechanical properties.
In view of the above, alternative composite compositions have been considered. In an article entitled “New Injection Moldable Electrostatic Dissipative (ESD) Composites Based on Very Low Carbon Black Loadings,” Journal of Electrostatics 47 (1999), p. 201-214, polymer-based composites are formed by combining polymeric materials with glass fibers and carbon black. The article, authored by Narkis et al., reports resistivities of as low as about 106 Ohm×cm when the composite contains a single polymeric material (polypropylene), glass fibers and carbon black. WO98/20503 to Narkis et al. discloses producing electrostatically dissipative polymer-based composites from thermoplastics modified with carbon black and glass fibers. WO98/20503 reports composites were produced having a matrix of a first thermoplastic material (e.g., polypropylene) and containing a second thermoplastic material (e.g., polyamide) and a dispersion of carbon black and glass fibers. According to WO98/20503, resistivities of as low as 0.1 Ohm×cm are possible if the second thermoplastic material has a higher polarity than the first thermoplastic material, such that the second thermoplastic material has an affinity for the glass fibers and the carbon black have an affinity to the second thermoplastic material, resulting in the carbon black forming an electrically conductive network within the matrix along the surfaces of the glass fibers. However, the lowest reported resistivity is 2 Ohm×cm.
It would be desirable if other methods were available by which carbon fiber-filled composites can be produced that exhibit improved electrical properties and suitable mechanical properties for electrical applications.